The present invention relates to wireless communication systems, and in particular, to systems and methods for synchronizing wireless communication systems.
Wireless communication systems are electronic systems that allow information to be transferred between two systems using electromagnetic waves propagating through space or air. FIG. 1A illustrates a typical wireless communication system. Wireless communication system 100 includes two wireless communication devices 101 and 102. In a typical wireless communication system, digital information is received in a first wireless device 101 and transmitted to the other wireless device 102. Similarly, digital information may be received in wireless device 102 and transmitted to wireless device 101. The transmitted digital information may then be used by other electronic devices coupled to wireless devices 101 or 102. Wireless device 101 may include a digital processing component 110, an analog processing component 120, and an antenna 121. During transmission, digital information may be received by the digital processing component 110 (e.g., from an electronic device such as a computer system). During reception, information may be received from analog processing component 120 as a digital data stream from an analog-to-digital converter, for example. Digital processing component 110 may perform a variety of functions for transmitting and receiving data including, for example, Fourier Transforms, Inverse Fourier Transforms, coding or decoding (e.g., for error correction), and a variety of other processing steps that occur on received and transmitted digital signals. During transmission, digital signals are transferred to an analog processing component 120. Analog processing component 120 may include analog-to-digital and digital-to-analog conversion, filtering, gain control, and frequency up coversion and down conversion, for example, to translate the digital information into an analog waveform that may be transmitted on antenna 121. The analog waveform is received by the other antenna 131 and processed by analog processing component 130. The received waveform may be amplified, down converted, filtered, and converted back into the digital domain. Digital processing component 140 in the receiving system may receive streams of digital information to be processed and provided as an output of the wireless system.
One problem associated with wireless communication systems involves synchronizing the transmitting and receiving systems so information may be accurately transmitted and received. For example, the data formats of the transmitted information may include precise timing characteristics. Additionally, processing the digital streams of data that flow from the analog processing component (the “analog front end”) may require that the digital processing component configure the analog front-end in particular ways at very precise periods of time. For example, FIG. 1B illustrates one wireless transmission scheme know as frequency hopping. In a frequency hopping wireless system, transmitted data may be modulated at different frequencies at different times. For example, data may be transmitted on a frequency channel 101 by modulating the analog signals at a frequency of f1. At other times, data may be transmitted on frequency channels 102 and 103 by modulating the analog signals at frequencies of f2 or f3. In a frequency hopping system, data transmission may move from one frequency to the next at certain points in time. In order to receive the transmitted data, the receiving system must accurately track the changes in frequency of the transmitting system. Thus, the receiving system must be synchronized in time with the transmitting system so that, for example, when the transmitting system changes from one frequency to another, the receiving system changes frequencies at the same time. Moreover, the receiving system must have some method of determining the frequencies that the transmitting system is hopping between. Other timing requirements may be associated with the digital formats of the transmitted data (e.g., packet formats) and the modulation techniques used. Synchronization of the transmitting and receiving systems becomes particularly important as data rates increase.
FIG. 2 illustrates a wireless communication system protocol. The example data transmission format shown in FIG. 2 illustrates some of the problems solved by embodiments of the present invention. The data transmitted from a wireless system may be used to perform a variety of functions. The structure of a data packet may include a preamble 201 used for synchronization, a second preamble 202 used for channel estimation and calibration, and then a payload carrying data. The synchronization preamble may comprise multiple blocks of data as shown at 220. In one example system, the synchronization preamble may include 24 blocks. Each block may also be referred to as a “symbol.” The symbols are transmitted sequentially by the transmitting system and received by the receiver. The transmitting system may send each symbol using a different frequency. For example, the first symbol, S1, may be transmitted on frequency f1, the second symbol, S2, may be transmitted on frequency f2, and the third symbol, S3, may be transmitted on frequency f3. The fourth symbol, S4, may be transmitted on frequency f1 again. This is an example of a frequency hopping pattern wherein the frequency hopping sequence is [f1, f2, f3]. Wireless systems may use a variety of other hopping sequences (i.e., hopping patterns).
The time domain illustration of a symbol is illustrated at 230. The start of the symbol is at t1. A time t3, symbol 230A ends and symbol 230B begins. Thus, times t1 and t3 represent the time domain boundaries of the symbol. Some protocols may include a division of the symbol into two components. The first component includes a data transmission component from time t1 to t2, and the second component of the symbol from t2 to t3 may be zero. If a total of 165 samples of a received symbol are obtained, 128 samples may include data and 37 samples may be zero, for example. For synchronization, the symbols may be a pseudo-random number sequence (e.g., “PN sequences” or “PN codes”). The data represented by the samples may be an orthogonal frequency division multiplexed (“OFDM”) signal, for example, as shown at 240. The baseband OFDM signal illustrated may include 128 subcarriers spread out between −264 MHz to +264 MHz. Of the 128 subcarriers, 112 may be spaced symmetrically about zero frequency, with 56 subcarriers such as fS1, fS2, though fSN, both above and below zero frequency. Each subcarrier may contain 2-bits of data, for example. Data may be encoded in the OFDM signal 240 and transmitted as a symbol during the period from t1 to t2. For a 264 MHz bandwidth OFDM symbol, the system may be sampled at 528 MHz. Accordingly, the symbol period (from t1 to t3) may be about 312.5 nanoseconds (“ns”). Therefore, a wireless system receiving symbols may be required to switch from one carrier frequency f1 to another carrier frequency f2 synchronously with the transmitting system, for example, with a precision of about 1 clock cycle, or about 2 ns). In particular, the receiving wireless system may have to reconfigure analog component to receive different signals at different frequencies at different times. The timing of analog reconfiguration should be such that the analog receiver circuits are able to process incoming data. Accordingly, changes to analog circuits must be timed precisely with changes in the transmitted signals. More generally, the timing of analog and digital processing steps may require close alignment in time with timing characteristics of received data.
Thus, there is a need for improved synchronization in wireless systems. The present invention solves these and other problems by providing systems and methods for synchronizing wireless communication systems.